Passata is a cooked tomato concentrate. In the case of the recipe you
refer to, which calls for a small amount, you might prefer to substitute tomato
paste rather than going to the trouble of making a batch of passata.

There is also a growing body of thought that passata is an uncooked tomato concentrate. For more on this theory, you might like to check out What is Passata, Take II?.

In the meantime, one of our lovely readers has supplied her own recipe for what she assures us is a delicious cooked passata:

Blend the pancetta, carrot, onion, and celery in a food processor and mill until fully chopped.

Place the boneless pork shoulder into a pan with the olive oil, and add the mix of chopped ingredients. Add a cup of very dry red wine and a cup of tomato sauce, and cook until pork is tender and liquid reduced, about 1-1/2 hours. Remove the pork and use for anything you like.

Add 2 or 3 tablespoons of tomato paste at a time and cook until drying. Add more, and do this over and over until the entire can is incorporated.

The resulting passata will be brownish, shiny, and very concentrated, and can be used for many different dishes. It should be refrigerated in a glass container, and will keep for several weeks.